Abstracts of Symposium 55 and some papers from other related Symposia.

Pleistocene Climatic Changes Recorded by the Paleosols in Northwestern Bangladesh

M.Shafiqul ALAM. Geology and Mining Dept. Univ. Rajashi. 6205 Rajashi. Bangladesh, Roland PAEPE, Edward KEPPENS & Joseph HUS, Earth Technology Institute, Vrije Univer. Brussels. 1050 Brussels. Belgium.

In the north-western part of Bangladesh, detailed studies on paleosols in a thick Pleistocene alluvial sequence have been carried out. Based on macromorphological observations such as desratification, horizon formation, colour, biogenic-pedogenic features (i.e. rootlets or root traces, burrows, CaCO₃ nodules, Fe concretions) and soil structures. 10 buried paleosols have been recognized in the Pleistocene deposits. The odd numbered paleosols (i.e. GSI, GS3, GS5, GS7 and GS9) are designated as moderately developed Bt paleosols as inferred from their relatively finer textures, higher concentration of pedogenic soil carbonate nodules and lower abundance of iron oxides. The even numbered paleosols (i.e. GS2, GS4, GS6< GS8 and GS10) are called here strongly developed Bt paleosols, containing very few or no pedogenic carbonate. Moreover, the contain Fe oxide concretions, burrows and root channels. Both moderately developed and strongly developed paleosols occur in cyclic succession. Strongly developed paleosols occur in thick sediment sequences, which may indicate that the flood basin accretion rate was quite high as a result of stronger hydrodynamic conditions. They are a direct consequence of bigger flood events. The relatively thinner sediment sequences, in which the paleosols are moderately developed, have resulted as a consequence of consecutive smaller flood events under drier conditions. Hence, the 10 buried paleosols suggest that at least 10 alternating dry and wet climatic phases prevailed during the successive phases of both paleosol development and sedimentation cycles. These alternating dry and wet climatic cycles most probably influenced the strengthening or/and weakening of the paleo-summer monsoon. Alternating occurrence of strongly and moderately developed Bt paleosols in the Pleistocene deposits in NW Bangladesh implies an alternation between warm humid climates with a summer monsoon dominance and cold-dry climate with a winter monsoon dominance. Moreover, studies based on isotopic compositions of stable carbon and oxygen as derived from the analysis on pedogenic carbonate nodules, as well as from the measured bulk magnetic susceptibility in the paleosol sequences, are found to be in good agreement with the above statement on the dry and wet phases. Both ¹⁸O and ¹⁴C results of the pedogenic carbonates from moderately developed paleosols are heavier than those from the strongly developed paleosols. The bulk magnetic susceptibility of the paleosol sequences clearly shows a cyclic pattern which correlates well with the ¹⁸O and ¹⁴C shifts.

	Back to the Table of Content
	Back to the Abstracts list